The present invention relates to intrusion detection systems and, more specifically, to intrusion detection systems in which interruption of a beam of invisible radiation, as, for example, infrared energy transmitted from a transmitter to a receiver, generates an alarm signal.
In intrusion systems of the above type, the transmitter and receiver are generally mounted to opposed walls near the area to be protected so that a transmitted beam of radiation scans the protected area, which may be, for example, near the inside of the front door or windows of a premises. Intruders entering the protected area break the transmitted beam of invisible radiation. Photodetection circuitry within the receiver, responsive to the absence of the transmitted beam, actuates alarm circuitry.
Optimum alignment of the transmitted beam of radiation and the receiver is of critical importance in the above described type of intrusion detection system. The receiver generates an alarm signal in response to non-detection of the transmitted beam, but cannot discriminate between absence of the beam due to interruption by an intruder or due to non-alignment of the beam and the receiver. Accordingly, improper alignment of the receiver and the transmitted beam can result in false intrusion alarm signals.
Various procedures for aligning the receiver and the transmitted beam are currently used. For example, the receiver may be pivotally mounted to its supporting surface and pivoted until a basic alignment is achieved. Alignment lights may be used for this purpose. After the basic alignment is established, a cutoff tool must be used to determine if the alignment is optimum or marginal. The cutoff tool blocks a certain percentage of the beam receiving surface of the receiver, usually of the order of 75%, for preventing the receiver from detecting a portion of the transmitted energy directed towards it. An alarm signal generated by this partial obstruction indicates that the transmitted beam and receiver are not optimally aligned, and the alignment procedure must then be repeated and further refined. It is apparent that the above described alignment procedure is complicated, time consuming, and requires tools and equipment in addition to the operating components of the system itself.
It is an object of the present invention to provide an intrusion detection system of the above discussed type in which simplified means for aligning the transmitted beam with the receiver, and means for indicating when such alignment is marginal, are incorporated directly into the receiver, enabling alignment to be made without the use of any special alignment tools. It is a further object of the invention to provide an intrusion detection system in which the transmitted beam has a relatively narrow vertical component to prevent reflection of the beam off the ground surface to further facilitate alignment of the beam and the receiver.